This invention relates to methods and apparatuses for measuring a shape, and particularly to a shape measurement method and apparatus suitable for measuring a cross-sectional or three-dimensional shape of a specimen in a nondestructive and contactless manner, utilizing a signal that is generated from the specimen upon exposure to electromagnetic waves (rays) or charged particle beams.
In order to form a wiring pattern on a semiconductor wafer, a coating called resist is applied to the semiconductor wafer; a mask on which the wiring pattern is formed is then placed over the resist, and visible rays or ultraviolet rays are applied through the mask to expose the resist and transfer the wiring pattern thereto. Thus-formed wiring pattern has sloped portions liable to change in slope angle and/or in shape according to focus and exposure dose of visible or ultraviolet rays that are applied, and thus requires measuring and testing in three-dimensional shape of the pattern to form a high-precision wiring pattern. For the purpose of testing the pattern, a wafer could be cut to precisely measure a cross section thereof, which would however require extra processes and costs.
In view of these circumstances, methods for measuring a cross-sectional shape of a pattern in a nondestructive and contactless manner utilizing an electron microscope image have been proposed. For example, JP 61-290313 A (also published under JP 5-54605 B2, and corresponding to Japanese Patent No. 1,842,661) has proposed measurement of a cross-sectional shape performed by a combined use of the xe2x80x9cShape from shadingxe2x80x9d (as disclosed in Ikeuchi, et al., xe2x80x9cDetermining 3D Shape from 2D Shading Information Based on the Reflectance Map Techniquexe2x80x9d, IEICE Transactions, Vol. J-65-D, No. 7, pp.842-849, issued by The Institute of Electronics, Information and Communication Engineers, in July, 1982) method and the stereo matching method. The methodology disclosed in JP 61-290313 includes detecting feature points of a signal waveform detected by a secondary electron detector provided in an electron microscope, measuring an absolute value of a height of the cross section by stereo matching of the feature points, thereby obtaining a shape between the feature points by the xe2x80x9cShape from shadingxe2x80x9d method.
The Spectroscopic Critical Dimension Metrology as disclosed in J. A. Allgair, et al., xe2x80x9cImplementation of spectroscopic critical dimension (SCD) for gate CD control and stepper characterizationxe2x80x9d, SPIE proceedings, Vol. 4344, paper 57, 26th Annual International Symposium on Microlithography, issued by The International Society for Optical Engineering, in February 2001; is also known in the art. In this method, in order to prevent damage to a resist pattern that would be caused by irradiation with ultraviolet rays, a visible ray is applied to a specimen, and a reflected light spectrum from the specimen is compared with those corresponding to various three-dimensional shapes which have been stored in a database in advance, so that the three-dimensional shape of the specimen is extrapolated.
Other disclosures related to the above techniques may also be found for example in JP 7-27549 A, JP 2-247964 A (corresponding to Japanese Patent No. 2,716,997), JP 5-290786 A, JP 63-32314 A (also published under JP 7-122574 B2), and JP 1-143127 A (corresponding to Japanese Patent No. 2,650,281).
The use of the stereo matching method as described above would disadvantageously cause misalignment between corresponding feature points due to a low signal-to-noise ratio of an input signal, producing an appreciable error in measuring a three-dimensional shape. On the other hand, the SCD Metrology as described above requires data obtained by measurement to construct a database, and has a limitation placed on measurable patterns; i.e., line repetition structures/lattice patterns only can be measured by this method. In addition, outputs are not provided with stereoscopic representation, but in the form of numerical data (width and height of wiring, and slope angles), which would not be adequate to show a three-dimensional shape.
Furthermore, the existing techniques as above give no adequate consideration to measuring three-dimensional shapes of varied patterns using a single secondary electron detector.
The present invention is made in view of the aforementioned disadvantages, and it is an exemplified general object of the present invention to provide a shape measurement method and apparatus that can precisely measure a cross-sectional or three-dimensional shape of a specimen, without utilizing a matching process of feature points.
The present invention adopts the xe2x80x9cShape from shadingxe2x80x9d method to pick up more than one of candidates for cross-sectional shape or three-dimensional shape of a specimen, from which an appropriate shape conformable to an actual measurement result is selected as a measurement of the cross-sectional shape or three-dimensional shape of the specimen.
To be more specific, a shape measurement method according to one exemplified aspect of the present invention includes the steps of: applying one of an electromagnetic wave and a beam of charged particles to a surface of a specimen, using an irradiation unit that moves along an axis parallel to a scanning direction relative to the surface of the specimen; measuring a signal intensity of one of an electromagnetic wave reflected from the surface of the specimen and a beam of charged particles generated from the surface of the specimen as a result of irradiation from the irradiation unit; calculating a slope angle of the surface of the specimen at a position irradiated with one of the electromagnetic wave and the beam of charged particles on the basis of the measured signal intensity; determining candidates for cross-sectional shape of the specimen on the basis of the calculated slope angle; estimating a signal intensity of one of an electromagnetic wave that would be reflected from a surface having a cross-sectional shape of each of the candidates and a beam of charged particles that would be generated from the surface having a cross-sectional shape of each of the candidates if an angle of incidence of one of the electromagnetic wave and the beam of charged particles with respect to the surface having a cross-sectional shape of each of the candidates were changed to a specific angle of incidence different from an angle of incidence of one of the electromagnetic wave and the beam of charged particles applied to the surface of the specimen; comparing the estimated signal intensity with a signal intensity obtained by measurement performed when the angle of incidence of one of the electromagnetic wave and the beam of charged particles applied to the surface of the specimen is changed to the specific angle of incidence; and determining the cross-sectional shape of the specimen on the basis of a result of the comparing step.
In the shape measurement method as above, the irradiation unit may be designed to further move along an axis perpendicular to the scanning direction relative to the surface of the specimen, so that the irradiation unit applies one of an electromagnetic wave and a beam of charged particles to the surface of the specimen while moving relative to the specimen along the axis parallel to the scanning direction (x-axis) and along the axis perpendicular to the scanning direction or parallel to the longitudinal direction (y-axis). Moreover, when the cross-sectional shape (variations in thickness) of the specimen is determined on the basis of the result of tile comparing step, the determined cross-sectional shape is accumulated each time when the irradiation unit moves along the axis perpendicular to the scanning direction, so that a three-dimensional shape of the specimen may be determined on the basis of a result of the accumulating step.
In the above shape measurement method, preferable features include:
(1) prior to determining candidates for cross-sectional shape, the slope angle of the surface of the specimen at a position irradiated with one of the electromagnetic wave and the beam of charged particles may be calculated using the measured signal intensity and multiple parameters selected from various kinds of parameters relating to the cross-sectional shape of the specimen;
(2) in order to accurately determine the cross-sectional shape of the specimen, the multiple parameters to be used may be selected among those serving to reduce a difference between the measured signal intensity and the estimated signal intensity;
(3) in order to accurately determine the cross-sectional shape of the specimen, more preferably, the multiple parameters may be repeatedly selected until the difference between the measured signal intensity and the estimated signal intensity is reduced to a minimum;
(4) the candidates for cross-sectional shape that have been determined, the cross-sectional shape that has been determined, and the multiple parameters that have been selected may be stored in a database; and
(5) when the angle of incidence of one of the electromagnetic wave and the beam of charged particles applied to the surface of the specimen is changed to the specific angle of incidence, an angle of placement of the specimen may be changed while an angle of irradiation of the irradiation unit is fixed at a predetermined angle; or, to the contrary, an angle of irradiation of the irradiation unit may be changed while the angle of placement of the specimen is fixed at a predetermined angle.
According to the method as described above, a slope angle of the surface of the specimen is calculated from a signal intensity that is obtained by actually applying an electromagnetic wave or a beam of charged particles to the specimen. Based upon the calculated slope angle, more than one candidate for cross-sectional shape of the specimen is for example determined. Assuming that the electromagnetic wave or the beam of charged particles were applied to the surface having a cross-sectional shape of each of the candidates with an angle of incidence being changed to a specific angle of incidence different from that of the electromagnetic wave or the beam of charged particles actually applied to the surface of the specimen, a signal intensity that would be obtained from each of the candidates for cross-sectional shape of the specimen is estimated. The estimated signal intensity is then compared with a signal intensity obtained by measurement performed when the angle of incidence of one of the electromagnetic wave and the beam of charged particles applied to the surface of the specimen is changed to the specific angle of incidence. Based upon a result of the comparison, the cross-sectional shape of the specimen is determined by selecting a cross-sectional shape serving to reduce a difference between the measured signal intensity and the estimated signal intensity as a most probable cross-sectional shape of the specimen. Therefore, the cross-sectional shape of the specimen can be acquired using an absolute value of the height (or thickness; i.e., distance measured along z-axis) of the specimen, without using a matching process of feature points. Further, accumulation of cross-sectional shapes of the specimen obtained while the irradiation unit moves in the scanning direction, which is performed each time when the irradiation unit moves along the axis perpendicular to the scanning direction (or along the longitudinal direction; i.e., along y-axis) enables determination of a three-dimensionial shape of the specimen.
A shape measurement apparatus according to another exemplified aspect of the present invention includes: an irradiation unit that applies one of an electromagnetic wave and a beam of charged particles to a surface of a specimen, while moving along an axis parallel to a scanning direction relative to the surface of the specimen; a signal intensity measurement unit that measures a signal intensity of one of the electromagnetic wave reflected from the surface of the specimen and the beam of charged particles generated from the surface of the specimen as a result of irradiation from the irradiation unit; a cross-sectional shape candidate determination unit that calculates a slope angle of the surface of the specimen at a position irradiated with one of the electromagnetic wave and the beam of charged particles on the basis of the signal intensity measured in the signal intensity measurement unit, and determines candidates for cross-sectional shape of the specimen on the basis of the calculated slope angle; a signal intensity estimation unit that estimates a signal intensity of one of an electromagnetic wave that would be reflected from a surface having a cross-sectional shape of each of the candidates and a beam of charged particles that would be generated from the surface having a cross-sectional shape of each of the candidates if an angle of incidence of one of the electromagnetic wave and the beam of charged particles with respect to the surface having a cross-sectional shape of each of the candidates were changed to a specific angle of incidence different from an angle of incidence of one of the electromagnetic wave and the beam of charged particles applied to the surface of the specimen; and a cross-sectional shape determination unit that compares the signal intensity estimated in the signal intensity estimation unit with a signal intensity obtained by measurement performed in the signal intensity measurement unit when the angle of incidence of one of the electromagnetic wave and the beam of charged particles applied to the surface of the specimen is changed to the specific angle of incidence, and determines the cross-sectional shape of the specimen on the basis of a result of the comparison.
In the shape measurement apparatus as above, the irradiation unit may be designed to further move along an axis perpendicular to the scanning direction relative to the surface of the specimen, so that the irradiation unit applies one of an electromagnetic wave and a beam of charged particles to the surface of the specimen while moving relative to the specimen along the axis parallel to the scanning direction (x-axis) and along the axis perpendicular to the scanning direction or parallel to the longitudinal direction (y-axis). The cross sectional shape is determined as described above while the irradiation unit moves along the axis parallel to the scanning direction, and the cross-sectional shape determination unit can accumulate thus-determined cross sectional shape each time when the irradiation unit moves along the axis perpendicular to the scanning direction, so that a three-dimensional shape of the specimen may be determined on the basis of a result of the accumulated cross-sectional shapes.
In the above shape measurement apparatus, preferable features include:
(1) the cross-sectional shape candidate determination unit may use the signal intensity measured in the signal intensity measurement unit and multiple parameters selected from various kinds of parameters relating to the cross-sectional shape of the specimen to calculate the slope angle of the surface of the specimen at a position irradiated with one of the electromagnetic wave and the beam of charged particles;
(2) the cross-sectional shape determination unit may instruct the cross-sectional shape candidate determination unit to select the multiple parameters among those serving to reduce a difference between the signal intensity measured in the signal intensity measurement unit and the signal intensity estimated in the signal intensity estimation unit;
(3) the cross-sectional shape determination unit may instruct the cross-sectional shape candidate determination unit to repeatedly select the multiple parameters until a difference between the signal intensity measured in the signal intensity measurement unit and the signal intensity estimated in the signal intensity estimation unit is reduced to a minimum; and
(4) the candidates for cross-sectional shape that have been determined in the cross-sectional shape candidate determination unit, the cross-sectional shape that has been determined in the cross-sectional shape determination unit, and the multiple parameters that have been selected in the cross-sectional shape candidate determination unit may be stored in a database.
According to the apparatus as described above, a slope angle of the surface of the specimen is calculated from a signal intensity that is obtained by actually applying an electromagnetic wave or a beam of charged particles to the specimen. Based upon the calculated slope angle, more than one candidate for cross-sectional shape of the specimen is for example determined. Assuming that the electromagnetic wave or the beam of charged particles were applied to the surface having a cross-sectional shape of each of the candidates with an angle of incidence being changed to a specific angle of incidence different from that of the electromagnetic wave or the beam of charged particles actually applied to the surface of the specimen, a signal intensity that would be obtained from the candidates for cross-sectional shape of the specimen is estimated. The estimated signal intensity is then compared with a signal intensity obtained by measurement performed when the angle of incidence of one of the electromagnetic wave and the beam of charged particles applied to the surface of the specimen is changed to the specific angle of incidence. Based upon a result of the comparison, the cross-sectional shape of the specimen is determined by selecting a cross-sectional shape serving to reduce a difference between the measured signal intensity and the estimated signal intensity as a most probable cross-sectional shape of the specimen. Therefore, the cross-sectional shape of the specimen can be acquired using an absolute value of the height (or thickness; i.e., distance measured along z-axis) of the specimen by making use of a single unit for measuring signal intensity (signal intensity measurement unit), without using a matching process of feature points. Further, accumulation of cross-sectional shapes of the specimen obtained while the irradiation unit moves in the scanning direction, which is performed each time when the irradiation unit moves along the axis perpendicular to the scanning direction (or along the longitudinal direction; i.e., along y-axis) enables determination of a three-dimensional shape of the specimen.
Other objects and further features of the present invention will become readily apparent from the following description of preferred embodiments with reference to accompanying drawings.